1. Field of the Invention
The present invention is directed to a method and to an apparatus for the transmission of information in various carrier frequencies with a frequency hopping method that can be implemented, for example, in a mobile station and/or a base station of a mobile radiotelephone system.
2. Description of the Related Art
What is referred to as the frequency hopping spread spectrum system is known as a method for the transmission of data. What is thereby to be understood by a frequency hopping spread spectrum system is a system wherein a plurality of carrier frequencies are offered for the radio transmission of data, and the carrier frequency currently which is employed is changed at periodic intervals. Particularly given a time-division multiplex system (TDMA), a change of the carrier frequency can ensue after every time slot of the time frame of the time-division multiplex transmission. Such a frequency hopping spread spectrum system has advantages to the effect that the energy of the entire radio transmission is distributed over all carrier frequencies. This is particularly advantageous when a generally available frequency band such as, for example, the 2.4 GHz ISM (industrial, scientific, medical) band is employed. According to the applicable regulations (FCC part 15 in the USA), an upper limit for the maximally occurring energy per carrier frequency is defined for this frequency band in order to keep interference with other subscribers as low as possible. It is. prescribed for the frequency change that at least 75 different frequencies must be used within a time span of 30 seconds. Further, each frequency may be used for a maximum of 0.4 seconds in 30 seconds. All frequencies must be used equally distributed on time average.
24 time slots, respectively 12 for uplink and for downlink, are defined in a 10 ms frame in the Digital Enhanced Cordless Telecommunications (DECT) standard. The FCC part 15, however, only makes a bandwidth of less than 1 MHz available in the ISM band. In order to meet this requirement, the plurality of time slots was reduced to 12 time slots in a 10 ms time frame, i.e. respectively 6 time slots for uplink and for downlink.
With 6 time slots for each direction and retaining the DECT time frame of 10 ms, each time slot would exhibit a length of 833 μs. The time s slots in the DECT standard have a length of 417 μs. Given a slow frequency hopping system, an inactive DECT time slot of 417 μs is required between two neighboring, active time slots wherein data is transmitted. In such systems, only 6 active time slots are respectively employed for data transmission in each direction. If such systems that work on the basis of a slow frequency hopping are also to meet the requirements of the FCC part 15 in the ISM band, an inactive blind time slot of 417 μs must in turn be present between neighboring active time slots. This blind time slot thus has half the length of a full time slot of 833 μs, as a result whereof—when a base time frame of 10 ms is retained—four active time slots are offered in each frame for the respective uplink and downlink, a blind time slot being respectively transmitted between them. The four active time slots have a respective length of 833 μs, whereas the blind time slots comprises a respective length of 417 μs. Given this structure, the frequency programming for the frequency hopping in the next, following active time slot can continue to be implemented at the end of the preceding active time slot. The programmed start frequency in the next active time slot can thereby be set during the blind time slots.
an advantage of the frequency hopping spread spectrum system is that the system becomes more insensitive to disturbances due to the offering of a great plurality of carrier frequencies. Over and above this, the security against tapping by third parties is enhanced in the system, since the third party generally does not know the carrier frequency to which a switch is made after a certain time span.
The sequence of carrier frequencies that are successively employed for the transmission is determined by an algorithm. Such an algorithm is identically implemented in the fixed station as well as in each mobile station of the mobile radiotelephone transmission. When, thus, a mobile part is synchronized with the appertaining fixed station, the mobile part and the fixed station undertakes the carrier frequency change predetermined by the sequence of the algorithm synchronously with one another.
Problems occur when the plurality of usable carrier frequencies is not temporally constant. This, for example, is the case when a carrier frequency recognized as disturbed is blocked during a certain time span and, thus, is not enabled for employment and, for example, is enabled for re-employment after a certain time span. Even given such a plurality of carrier frequencies fluctuating over time, it must be assured that, for example, the aforementioned FCC part 15 rules are adhered to.
European Patent Document EP-A-0 182 762 discloses a method in a telecommunication system having two transmission/reception stations that selects carrier frequencies according to the frequency hopping method, whereby new carrier frequencies are selected from a matrix with available frequencies by generating a sequence of random numbers that reference the position of a respective carrier frequency in the matrix and on the basis of status information for the respective frequency likewise stored in the matrix, so that they can be read out in a next step.
U.S. Pat. No. 5,471,503 discloses a method for sampling a reception signal in a telecommunication working according to the frequency hopping method, whereby each channel checks for an existing transmission.